Motors are widely used in CPU fans. The motors can be classified into DC (direct current) motors and AC (alternating current) motors according to their power supply.
FIG. 1 shows a prior art DC brushless motor. The prior art DC brushless motor in FIG. 1 comprises a rotor which may be a bar permanent magnet and a stator which may be a coil. In order to sense the position of the rotor, there will be a position sensor. The position sensor may be a hall sensor. According to the right-hand rule, when the current Ic flows through the coil 101 with the direction shown in FIG. 1, there will be a electromagnetic field with an N (north) magnetic pole above and an S (south) magnetic pole below. According to the law of opposite magnet poles attract but like magnet poles repel each other, the magnetic torque generated by the electromagnetic field will force a counterclockwise rotation spin of the rotor 102.
The rotor 102 is free to rotate about its center, but is otherwise fixed. In FIG. 1, each end of the rotor 102 experiences an equal but oppositely directed radial force. ω represents the angle between the magnetic pole of the rotor 102 and the magnetic pole of the coil 101. If the rotor 102 rotates slowly, it will have the tendency to come to rest in the aligned position at ω=0° or ω=180°. That is, as the magnet rotates, it will experience a force that will try to align the rotor 102 with the magnetic pole of the coil 101 as shown in FIG. 2. When the rotor 102 is in the aligned position, there will be no magnetic torque upon the rotor 102. When the rotor passes the aligned position because of inertia, the direction of the magnetic torque upon the rotor 102 will change and the magnetic torque will force the rotor 102 to back to the aligned position if the direction of the current flowing through the coil 101 remains unchanged.
In order to keep the rotor 102 rotating with fixed direction, for example, counterclockwise, the direction of the current in the coil 101 should be changed after the rotor passes the aligned position. Normally, the direction of the current flowing through the coil 101 changes in response to the position of the rotor which is sensed by the position sensor.
FIGS. 3 and 4 schematically show how the position sensor 103 senses the relative position of the rotor 102 to the coil 101. The position sensor generates a sense signal Vsense based on the angle w. When the angle ω is between 90° and 270°, the sense signal Vsense is logical low. Otherwise, the sense signal Vsense is logical high. The direction of the current flowing through the coil 101 changes in response to the sense signal.
FIG. 5 shows an equivalent circuit model of the DC brushless motor in FIG. 1. The equivalent circuit model comprises an equivalently parasitical inductor Lp, an equivalently resistor Rp and an induced electromotive force Vt, wherein the induced electromotive force Vt is generated by the rotation of the rotor 102. According to Lenz's law, the magnitude of the induced electromotive force Vt is proportional to the product of the speed of the rotor and the variation rate dB/dt of the flux density. When the rotor 102 is close to the aligned position, the variation rate dB/dt of the flux density decreases, causing a decrease of the induced electromotive force Vt.
FIG. 6 schematically shows a DC brushless motor system which comprises a power supply and the equivalent circuit model of the DC brushless motor. The power supply may comprise a full-bridge inverter or a half-bridge inverter. In the example of FIG. 6, the power supply comprises a full-bridge inverter including a first bridge and a second bridge. A first switch 14 and a second switch 15 are series coupled to constitute the first bridge which is coupled between the input signal VIN and the ground. A third switch 16 and a fourth switch 17 are series coupled to constitute the second bridge which is coupled between the input power VIN and the ground, too. The equivalent circuit model of DC brushless motor has a first terminal A and a second terminal B, wherein the first terminal A is coupled to the conjunction of the first switch 14 and the second switch 15, and the second terminal B is coupled to the conjunction of the third switch 16 and the fourth switch 17. When the first switch 14 and the fourth switch 17 are turned ON, and the second switch 15 and the third switch 16 are turned OFF, a current is flowing in the direction shown (dashed line) in FIG. 6. The voltage across the first switch 14 and the fourth switch 17 is low and could be ignored, thus the voltage VAB between the first terminal A and the second terminal B is equal to the input signal VIN. When the rotor 102 rotates close to the aligned position, the sense signal Vsense decreases, so does the variation rate dB/dt of the flux density and the induced electromotive force Vt. As a result, the voltage VLp across the equivalently parasitical inductor Lp and the equivalently resistor Rp increases. Correspondingly, the current ILp flowing through the equivalently parasitical inductor Lp increases. When the rotor 102 is in the aligned position, the variation rate dB/dt of the flux density decreases to zero, and the current ILp flowing through the equivalently parasitical inductor Lp rises to a maximum value, thus there will be a current spike in the current ILp, as shown in FIG. 7.
To keep the rotor 102 rotating with fixed direction, the direction of the current of the coil 102 should be changed after the rotor 102 passes the aligned position. At the moment when the rotor 102 is at the aligned position, the second switch 15 and the third switch 16 are turned ON, and the first switch 14 and the fourth switch 17 are turned OFF. Because the current of the inductor could not change instantaneously, the direction of the current ILp maintains for a short period of time, and the current ILp passes through the second switch 15, the equivalently parasitical inductor Lp, the equivalently resistor Rp, the induced electromotive force Vt, and the third switch 16, and back to the input power VIN. The backflow current to the VIN causes voltage spike upon the input capacitor CIN (not shown in FIG. 6), shown in waveform VCIN in FIG. 7.
From the above description, the changing of the direction of the current flowing through the coil when the rotor is in the aligned position causes a current spike and a voltage spike. The present disclosure provides a control strategy for DC brushless motor driver to decrease the current and voltage spikes.